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At the local Trader Joe’s they hide stuffed animals amidst the groceries—and the lucky child who finds one wins a strip of fruit leather.

The rattenkönig was something of a medieval variant of this little game. Only instead of a cuddly stuffed animal, the item in question was a grotesque bundle of rats tangled together in a ghastly lump of broken, knotted tails and congealed filth. And if you found it hidden under a floorboard or between the walls of your European home? Well, the prize wasn’t so much fruit leather as it was the ravages of Black Death.

In an episode of The Lair of the RatKing that went up last year, I mentioned that this ominous über-vermin is largely considered cryptozoological. While rat king specimens pop up in museums from time to time, most assume human fingers are responsible for all that disgusting knotting.

Yet the natural world is hardly devoid of tail-entwined monstrosities—we just have to look to the larval cercaria stage of the parasitic trematodes. These tiny flatworms use vertebrates as their definitive host and molluscs as their intermediate host. So they have to return to something like a duck in order to complete their life cycle and produce eggs.

But how to return to a duck when you’re just a tiny, wiggling cercaria out there in the water? Well, in five trematode super families they go the rat king route, tangling their tails to form clusters of several to hundreds. By tangling together like this, the tiny parasites are more easily mistaken for food by that hungry duck. Plus, while individual cercaria swim poorly, aggregated members all rotate in the same direction to move their rat king form through the water.

Granted, the advantages of cercariae tail entanglement don’t really translate into the rat world. Rats don’t need to be ingested to complete their life cycle and tangled tails certainly don’t improve mobility. But that’s the lesson, isn’t it? What’s grotesque and senseless in one corner of the animal kingdom is an evolutionary advantage in another.

If you want a nifty artistic interpretation of the Rattenkonigcercariae, check out this illustration on DeviantArt.

Monster of the Week is a — you guessed it — regular look at the denizens of our monster-haunted world. In some of these, we’ll look at the possible science behind a creature of myth, movie or legend. Other times, we”ll just wax philosophic about the monster’s underlying meaning. After all, the word “monstrosity” originates from the Latin monstrare, which meant to show or illustrate a point.